When a vehicle, for example a dump truck, operates on a site with long, high speed downhills, it can get into situations where the vehicle powertrain enters an overspeed condition. An overspeed condition can also occur on a very steep downhill, where the operator does not have full control over the vehicle speed. The manufacturers of these vehicles apply various methods to try and avoid these overspeed conditions from occurring. The powertrain usually refers to the engine and transmission. The drivetrain usually refers to the group of components other than the powertrain that deliver engine power to the road surface, which can include for example the driveshafts, differentials, final drive and wheels. In this disclosure, powertrain will be used to refer to both the vehicle powertrain and the vehicle drivetrain.
The methods that are most commonly utilized to avoid these overspeed conditions are to remove throttle control from the operator, and apply an engine brake (engine valve brake, exhaust brake or both) or a transmission retarder in an effort to slow the vehicle down. In vehicles that do not have transmission retarders, the manufacturer might utilize the vehicle's service brakes in an attempt to slow the vehicle down.
These techniques may be successful in many cases, but in instances where there are extended high speed downhill operations, these measures may not be sufficient to cope with the high energy demands that are required for the continued retardation of the vehicle. It would be desirable to have additional retardation capacity available for use in cases where this capacity is beneficial or necessary to avoid or regain control from an overspeed condition.